In displacement detecting apparatuses, for instance, optical linear encoders, there is an apparatus that detects a fine displacement quantity as in a torque sensor and the like, which uses a deformation of an elastic body, in addition to an apparatus which detects a comparatively large displacement quantity of cm order, through a relative displacement quantity between a scale and a detecting head.
In such a type of torque sensor, any one of the scale and the detecting head is mounted on the elastic body, and the detecting head or the scale is mounted on a fixed member arranged in the vicinity of the elastic body so that the torque sensor can detect a torque which is applied to a disk and/or cylindrical elastic body, based on deformation of the elastic body in a rotational direction. Such a displacement detecting apparatus of the torque sensor has, for instance, a structure illustrated in FIGS. 3A and 3B of the following PTL 2. The displacement detecting apparatus in PTL 2 measures a fine deformation quantity of the elastic body, by an optical linear encoder which is provided on a circumference that is distant from the center of the rotation of the elastic body at a fixed distance.
In such a linear encoder, an optical pattern is provided in a scale, which has a lattice arrangement that is formed according to light and dark, a change of reflectivity, or the like. In addition, light-receiving elements of the lattice arrangement, which detect the optical pattern of the scale, are arranged in the detecting head. In addition, the torque sensor of this type, which uses the linear encoder, is occasionally used in order to detect a driving torque of a joint of a robot apparatus.
In this case, the torque sensor having the structure illustrated in PTL 2 detects the displacement quantity of an elastic body while regarding initial positions of the scale and the detecting head as starting points, and accordingly occasionally misses an absolute value of the displacement quantity (torque) due to a cause such as a crash. In such a case, it may become necessary to stop the operation of the robot apparatus, to disassemble the apparatus depending on the case, and to align the initial positions (zero point adjustment) in some cases. In order to eliminate this zero point adjustment and enable the apparatus to be operated without being stopped and disassembled even when the apparatus has caused the crash, the displacement detecting apparatus of the optical linear encoder type needs to be capable of detecting the absolute quantity of displacement, in other words, absolute displacement.
The phrase “detecting absolute displacement” described here means to detect a “relative” displacement quantity between the scale and the detecting head, as the absolute quantity, through an output of a detecting head without a calibration of a zero point (relative initial positions of scale and detecting head). For instance, the torque sensor having such a basic structure as in PTL 2 can acquire an incremental displacement quantity from the detecting head, but cannot detect the absolute displacement unless the relative initial positions of the scale and the detecting head are correct.
Here, PTL 1 discloses a structure for detecting the “relative” displacement quantity between the scale and the detecting head, as the absolute quantity. The encoder in PTL 1 arranges an absolute displacement track having a lattice arrangement of which the spacing is rough, and an incremental displacement track having a lattice arrangement of which the spacing is fine, in the scale. In addition, the encoder has each of a detecting head which detects light reflected from the absolute displacement track, and a detecting head which detects light reflected from incremental displacement track provided therein. By having such a structure, the encoder can obtain an absolute displacement value of a high resolution, by combining an absolute displacement signal which is obtained from the detecting head of the absolute displacement track, with an incremental displacement signal which is obtained from a head for detection of the incremental displacement track.